The present disclosure relates to a microfluidic element for analysis of a liquid sample and, more particularly, a microfluidic element for analyzing a fluid sample, with a substrate and a microfluidic transport system having a channel structure enclosed by the substrate and a covering layer. The channel structure comprises a channel and a chamber in fluid communication with the channel. The fluid is transported through the channel into the chamber, whereby entry of the fluid into the chamber is carried out in a controlled manner.
Microfluidic elements or test carriers of this type are used, for example, for biochemical assays in which various parameters of the fluid are assayed in the chambers. Tests of this type are used in in-vitro diagnostic systems, for example for immunological assays. These immunological tests often require a multi-step reaction protocol, so that the test procedure is carried out in a plurality of sub-steps. As an example, the sample to be tested is initially placed in the sample chamber. There it will be brought into contact with immobilized receptor molecules, i.e., molecules that are locally fixed in the chamber, so that the molecules in the sample fluid that are complementary to the receptor molecules can react with them. These receptor molecules may be in the form of individual spots, but they may also be immobilized in a microarray. A microarray is advantageous when various sample parameters are to be tested in one chamber. After reacting the sample with the immobilized receptor molecules, which may be antibodies, for example, in a further step of the procedure, the sample chamber is washed with a washing fluid. In a next step, a labeling fluid is transported into the chamber or reagents are added so that detecting antibodies can reach the bound molecules. Labels of that type may, for example, be receptor antibodies with fluorescent tags. In a further step, washing of the chamber is again carried out using a washing fluid. This step is, for example, used for “bound/free separation”, in order to separate free and unbound detecting antibodies, for example antibodies coupled with a fluorescent label. This washing process is frequently carried out several times in a row so that all of the free labeled antibodies can be removed. Only in this manner is it guaranteed that all free antibodies have been removed and only the bound antibodies are measured.
Due to the nature of the system and because of the lack of space it is often not possible with microfluidic elements to integrate a separate channel with a separate capillary stop into the test carrier or the microfluidic element for each step of the process. Thus, the channels and valves have to be used a plurality of times in succession. In order to control the fluid flow, geometric valves, for example, are used as a capillary stop. Fluid transport by capillary action is stopped by the abrupt change in cross-section when the small channel opens into the larger chamber. This transition thus forms a valve.
Valves and transitions that are known in the art, however, are designed to be used only once. For multiple use, they cannot be vented in a reliable and robust manner, and so controlled fluid transport cannot be reliably ensured. Particularly in the case of washing buffers, which comprise solutions containing detergents, a soap film often forms at the valve, so that venting of the valve and the channel is prevented by the soap film. Furthermore, due to the nature of the system, high capillary forces or adhesion often lead to fluid residues being left at the edges or corners of the channels. Thus, during capillary filling, for example of a siphon valve (S- or U-shaped channel between two chambers), fluid residues remain at its end which, when passing over into the next chamber, can flow into each other and thus air can no longer escape during subsequent filling. Filling with fluid stops and transfer of the fluid into the next chamber is thus no longer possible, since the principle of communicating channels (pipes) is no longer satisfied. This risk is particularly high with the microfluidic channels used, which comprise a siphon structure and are supposed to be used multiple times. In other valve types, residual fluid in the channels is less critical, since in that case, communicating channels are not required for the channel structure to function. The problem principally arises where channels have to be filled by capillary action.
Various approaches to a solution have been published in order to provide a valve that can be used multiple times. As an example, U.S. Pat. Appln. Pub. No. 2007/0134799 A1 and U.S. Pat. No. 6,395,553 B1 propose a microfluidic valve wherein a spring-loaded steel ball closes an outlet. A valve of that type, which is expensive and complicated to produce, is used in microfluidic test assays when a siphon-like structure cannot be employed for the channels. In order to transport the fluid, it is necessary for the valve at the chamber outlet to be opened. This is usually accomplished by generating a centrifugal force, and so the use of such a valve is limited to rotary test carriers.
As rotation begins, the ball inside the valve is forced radially outwardly and opens up the orifice so that the fluid can flow through the valve. When the speed is less than a pre-set rotational speed, the centrifugal force is reduced and the spring force of the spring acting on the ball predominates and the valve is closed. In addition to the spring force, in order to open the valve, the friction of the valve ball must also be overcome.
The forces required to control the fluid are frequently also produced in other capillary structures by centrifugal forces. However, other means for controlling fluids are known, as summarized, for example, in DE 10 2005 048 260 A1. One possibility for controlling the fluids in rotary test carriers is the use of a siphon channel between two chambers, wherein the desired fluid control is obtained by an appropriate arrangement of the inlet to the siphon channel and the outlet therefrom in the radial direction. Int. Pat. Appln. Pub. Nos. WO 95/33986 A1, WO 95/06870 A1, WO 93/19827 A1 and U.S. Pat. No. 5,160,702 B1, for example, also employ a concept of that type.
Despite the measures taken in the references cited above to improve microfluidic structures for the purposes of biochemical assays, problems arise again and again with venting the individual channels, in particular the siphon channels. These problems increase with multiple use of a channel by various fluids, in particular with siphon channels, which are relatively narrow and are in the region of below approximately 0.4 mm.
Thus, there is a need in the art for the provision of a microfluidic element with a microfluidic channel structure in which on the one hand, reliable control of fluids inside the channel structure is obtained even upon multiple use, and on the other hand venting is reliably carried out when in multiple use. In addition, such an element should be inexpensive to produce.